1. Field of the invention
The present invention relates to a fixing device mounted on image-forming machines such as a copier of the type of electrostatic photography, a printer and a facsimile. More specifically, the invention relates to a fixing device which includes a thermally fixing roller, an endless belt, and pushing means which pushes the belt toward the thermally fixing roller from the side of the inner peripheral surface of the belt such that part of the region in the circumferential direction on the outer peripheral surface of the belt is pushed onto part of the region in the circumferential direction on the outer peripheral surface of the thermally fixing roller, wherein the belt is driven when the thermally fixing roller is driven to rotate.
2. Description of the Related Art
As a fixing device mounted on an image-forming machine, there has heretofore been widely used the one of a form including a thermally fixing roller that is heated by a source of heat and a pressing roller that is brought into pressed contact with the thermally fixing roller. In a fixing device for a color image-forming machine that is finding a widening application in recent years, however, toners of, for example, four colors must be fixed in an overlapped manner and, hence, fixing property must be improved as compared to that of the fixing device used for the monochromatic image-forming machines. One of the means for improving the fixing property may be to increase the thickness of the elastic layer provided on the thermally fixing roller and/or on the pressing roller, such as increasing the thickness of the elastic layer of the pressing roller or forming an elastic layer on the surface of the thermally fixing roller, in order to increase a nipping width between the thermally fixing roller and the pressing roller.
However, it is a new trend to decrease the thickness of the elastic layer of the thermally fixing roller as much as possible to meet the countermeasure for saving energy on a global scale in recent years as well as to meet the user's requirements for shortening the warming-up time of the fixing device and saving the consumption of electric power. If the thickness of the elastic layer of the thermally fixing roller is decreased as much as possible, however, the nipping width decreases between the thermally fixing roller and the pressing roller, and the fixing property is spoiled.
In view of the above technical background, there has been developed a fixing device equipped with an endless belt mechanism instead of the pressing roller. A representative example of the fixing device of this kind may be the one which comprises a thermally fixing roller, an endless belt, and pushing means which pushes the belt toward the thermally fixing roller from the side of the inner peripheral surface of the belt such that part of the region in the circumferential direction on the outer peripheral surface of the belt is pushed onto part of the region in the circumferential direction on the outer peripheral surface of the thermally fixing roller, wherein a nipping region is formed between part of the region of the belt and part of the region of the thermally fixing roller that come in contact with each other, and the belt is driven when the thermally fixing roller is driven to rotate. The pushing means includes an upstream support roller arranged on the upstream side of the thermally fixing roller in the direction of rotation, a downstream support roller arranged on the downstream side in the direction of rotation, and a spring mechanism for pushing the upstream support roller and the downstream support roller onto the thermally fixing roller via the belt (see JP-A-2004-212844).
According to the above belt-type fixing device, the nipping width between the belt and the thermally fixing roller can be increased yet decreasing the thickness of the elastic layer of the thermally fixing roller as much a possible, and good fixing property can be accomplished. Though the above advantage is obtained, however, a too increased nipping width of the belt relative to the thermally fixing roller causes the paper to move along the curvature of the thermally fixing roller for an extended period of time and, hence, to be excessively heated developing such inconvenience that the water content contained in the paper is excessively evaporated forming a defective image (white spots), the paper is poorly parted from the thermally fixing roller, the paper is curled, etc. The paper can be effectively parted by arranging peeling pawls and by brining the peeling pawls into contact with the thermally fixing roller. When the above countermeasure is put into effect, however, wear increases on the surface of the thermally fixing roller, scars occur, life of the fixing device is shortened, and it is forced to replace the fixing device at the time of maintenance of the image-forming machine. Besides, the toner and the paper dust stay between the thermally fixing roller and the peeling pawls to contaminate the surfaces of the paper.